Screw rolling die and method



Feb. 12.,L 1934.

l.. AQ PRAYER SCREW ROLLING DIE AND METHOD Filed Feb. 15. 19.32 ssheets-sh t 1 Feb. 13, 1934. 1 A. FRAYER SCREW ROLLING DIE4 AND METHODFiled Feb. 15. 1932 I5 Sheets-Sheet 3 Patented Feb.V 13, 1934 l1,946,735 SCREW ROLLING DIE AND METHOD Lee A. Frayer; Ravenna. Ohio,assigner to Roy 11.` Smith, Kent. Ohio Application Febrpary 15, 1932.Serial No. 593,010

-20 Claims.

This invention relates to the formation of screw threads by rolling ablank between suitably shaped dies, and its generalobject is to producework of uniform size from blankmaterial which is not uniform in size ordegree of heat plasticity or malleability.

In the cold-rolling of screws, the method commonly employed involves theuse of blanks with threading portions of less than the ultimatescrew-thread diameter, which must be accurately made of determinatediameter and hence require a preliminary turning or equivalent operationon the blank, and dies which work simultaneously from both sides overthe entire length of the screw, the metal being depressed from the blankdiameter to form the inner portions of the thread groove, and raisedbeyond thev blank diameter to form the outer portions of the thread. Inthis method there is no elongation of thesblank, and the depthpf threadobtainable by cold rolling limits the size of work which can beproduced.

The hot-rolling of screws is commercially practiced to a small extent byforcing a heated blank endwise between a set of die rollers whose axesare tilted and set at an angle to the axis of the blank, thereby formingthe thread progressively .from the tip toward the head end of the blank,and

elongating the blank during the process. This,

method is not adapted to the formation of wood screws with pointed tipsnor can it be satisfactorily used with substantially undersizedmaterial.

My present invention involves forging or swaging of the thread byworking toward or to the tip end of the blank, accompanied by elongationor extrusion of the blank in that direction, and is adapted both to theforming and pointing of.

wood screws orthe making of .screws having a standard or machine-screwthread. Methods resembling'this in some particulars have previously beenproposed, for example as long ago as the Beecher Patent No. 77,710 ofMay 12, 1868, employing either two roller dies or two flat dies whoseribbed portions are duplicates of each other, but so far as I am awaresuch devices have heretofore been unsuccessful, notwithstanding thepotential saving in production cost as compared with cold rolling yorcutting, and the wide field for stronger and cheaper screws havingthreads .of reasonable accuracy.

Having ascertained the causes of prior failures, which lie principallyin attempting too rapid an elongation of the blank and gathering asurplus of metal for the-initial portion of the thread which cannot beproperly worked into the substance thereof, with the result'of piping orsplitting the stock` or withdrawing of necessary metal from thepartly-formed thread, I have been able in the present invention toprovide a successful solution of this problem. Y c

It is of vital importance in progressively rolling 00 a screw threaduponfthe blank in the direction of its tip end, and incidentallyelongating the blank,

to avoid the accumulation of superfluous metal between the rst two dieribs in the initial formation of the thread, for such excess in thelater 05 stages mustbe squeezed out longitudinally of the thread, whichproduces a barrel-shaped screw or piping .ofthe stock caused by workingit in an oval shape and forcing the material onward as a shell without asolid integral core. 0n the other 'I0 hand, if the proper amount ofmaterial is gathered in the thread, but the attempt is made to form athread groove of the ultimate width andproille at the outsetl by meansof the die rib which first takes hold, it will be found that said ribdraws out 7l the thread metal which has just been raised and leaves ascant thread or none at all.

My invention ayoids these xdiiculties by providing dies of differentcharacteristics for working on opposite sides of the blank so that, forl0 example, the die on one side may progressively measure outsubstantially the correct volume or cross-sectional area of stock forthe thread, but leave it in a dierent shape or diiferent position orpitch spacing than it will ultimately BI assume, while the die on theother side may have the ultimate ypitch spacing and a groove shapewhichfwill re-form the thread left by the rst die eitler to its ultimateshape or to a shape which is then further re-formed by the ilrst die. 00The thread is thus given its ultimate shape and pitch spacing by the rsttwo ribs or ridges on one of the dies and the first rib on the otherdie, and subsequently-acting ridges and grooves which may be providedonboth ofthe dies have 9| no further effect in forming the screw but actmerely to guide it in order to preserve -its properly aligned positionbetween the dies.

Of the accompanying drawings, Fig. 1 is a face or plan view of one ofthe thread-rolling 10o dies embodying and adapted to carry out my.invention', in this case the lower dies shown in Fig. 5, developed intoa ilat surface, a wood screw being represented in four successive stagesof formation along,` the die, the' integral die. sur- 10i face beingdivided at the dot-and-dash line and shown in two' parts placed oneabove the other.

Fig. 2 is a similar developed view of the opposite or upper die withoutthe screw thereon.

Fiss. 3 and 4 are cross sections, each taken ire` through both dies andthe intervening work at the positions 3-3 and 4 4 on Fig. 1.

Fig. 5 is an end elevation showing part of a screw-rolling-machineequipped with the roller dies, with the work in section.

Fig. 6 is a side view of a portion of a screw as partly formed bymodified dies with a standard or machine screw thread in accordance withthis invention.

Fig. 7 is a detail section of a second modication showing thepinching-oir action of suitablyformed dies in finishing the end of .asharppointed wood screw.

While the screw-rolling dies might be nat and relatively reciprocated toact on the work in accordance with familiar practice, this would make anexcessivels7 long rolling machine for large work and it is preferred tomake them parts of die rollers 10, 1l, mounted one below the other onsuitable shafts as indicated in Fig. 5, and rotated in the samedirection at the same angular speed by means of suitablegearingconnected with their shafts as shown in broken lines. Theadjacent working faces of the dies move in opposite directions, and theyturn the work between them on a stationary axis. I prefer to employoverhanging die rollers, with shaft bearings on one side only.

12 and 18 are respectively the lower and upper dies proper, detachablyheld upon the roll bodies by suitable clamping devices. Thecircumferential length or each die is such as to leave preferably a Inof about 30 degrees of open space bet: Us ends in order to allow ampletime for i reduction of the blanks between the rolls while preservingthe desired surface speed of the latter for accomplishing the productionin a given time.

r"he work is laterally introduced fromy the iront or lethand side of themachine, as viewed in Fig. 5, with the aid of a reciprocating pusher i4suitably timed by the machinewith reference to the rotary positions ofthe dies. A nxed abutment or back-stop 14.n is provided on the far sideof the roll pass to limit the inserting moveM ment, but when. the dieshave taken hold, the work moves forward slightly from this abutment andis automatically held in the narrowest part of the roll pass by theaction of the dies themselves.

The ribbed thread-forming surface or' one of the dies, the lower one l2in this case, is angularly advanced'with respect to that of the otherdie 13 by an amount corresponding to a half rotation of the screw blank,so that the first rib on the lower die begins to operate on the work ahalf-turn of the latter ahead oi the first rib on the upper die, and thelower die preferably also finishes approximately one half-turn of thework before the upper die, so that the trailing end of vthe upper die 13lags behind that of the lower one, as seen in Fig. 5.

Advantage is taken of this trailing overlap of the upper die, in thecase of short work, in helping to eject the nished screw into a notch 15which is formed in the body of the lower die roller 10 parallel with theaxis of said roller and adja- Vcent the trailing end of the lower die,whereby the finished screw, after being dropped into the notch, may becarried around to the lower side of the lower die and discharged intowater in a cooling receptacle. In the case of long rods which requirethreading at several points along their length and are supported by asuitable work holder, or single-threaded rods so long that they wouldwhip beyond the rolls if not so supported,

the notch l5 is not utilized but the nnished work is withdrawn laterallyfrom between the die rollers oppositely to the. direction of itsinsertion.

16 indicates a cylindrical work blank or short rod which, by theoperation of the dies, becomes a finished screw, as seen at the lowerright in Fig. l, illustrating a railroad-spike wood-screw adapted to bescrewed into a hole drilled in the tie. The blank in such a case ispreviously provided with a head 17 which may be forged by an upsettingoperation on a heated section of the original rod. The screw body of thespike tapers down from the original diameter near the head and tapers toa blunt point at the tip or lower end, while the thread 26 is shallow atboth ends and of full depth in the intermediate portion of the screw.

Both dies are provided with a series of slanting ribs or ridges andintervening thread-forming grooves having an angle to the direction ofdie motion corresponding to that or" the screw thread to be formed. The.rst r ing into action on the lower die I 19a and begins at a point oreno'. of Said die, preferabl'" 20 of said die which cons ward edge onthe roller l0. ribabove the adjacent fiat s' is relatively small at thebe te a maxirnu. at about t of the thread groove, erf; of said rib atthe begin ln or front edge of the rib as gradually becomes parallelsurface 2i at about the thi that the thread groove correspond shallowand increases to its bottom forming the sc threads tapers from the in'irniun blank d eter to the intermediate gione-bottom dair and becomesfully cylindrical at about the groove turn. The ribs on the 'upper dierates o r' iivelent to those on the lower but hei 'ons er" proper heightare longitud reference to the corresponding *I allow for the fact thatthey come into the thread groove a half-turn or the work later than thelower-die rib portions. At its trailing end this rst lower-die rib 191 estill higher, and the first rib on the upper d.V correspondinglyelevated, as seen at the right in Figs. 3 and 4., to form the taperedpoint on the screw body.

The second rib on the lower die is numbered 19h, the third 19c and soon, while the intervening thread-forming grooves between the successiveribs are numbered 22a, 22h, 22C, etc. Each succeeding rib after thefirst one begins at a point further along the outer edge 2O of the die,and all of the ribs originating in this edge have pointed ends toprovide an easy entrance into the blank metal or into the formed th-readgroove. Each succeeding one, like the rst rib, is likewise shallowest atthe beginning and increases to a 1UP. maximum depth further along, andthe top face of each at the beginning is lowest on the side toward thedie edge 20 and becomes parallel with the plain die face 2l at about thefourth turn of the thread groove so that the successive ribs 145 afterthe rst one will properly thread into the groove. As the trailing endsof the rst, second and third ribs 19e, 153b and 19c are approached,their adjoining sides forming the intervening die grooves 22E, 22b and22c spread toward each othlo@ er, and the bottoms o! said grooves risetoward the tops of the ribs, so that the grooves taper on to nothing.The sides rof the first groove 22, and of the second one 22b to someextent, by working of the thread metal, correspondingly reduce theheight of the thread which tapers down to nothing near the screw tiD,1asseen at the lower right in Fig. 1. The tops of the second, third andfourth ribs 19, 19c and 19d also rise further toward their trailing endsso 'that the second rib will participate in the tapering of the pointand the third and fourth will follow the threadgroove bottom as thefinished screw point rolls along to the trailing end of the die.Obviously the dies could be modified in these respects, if desired, tocarry the thread clear to the end, forming an ordinary sharp-pointedwood-screw tip and pinching of! the excess metal, as shown in Fig. 7.

The opposing or upper die 13, whose development is represented in Fig.2, except for differences in its swaging portion, has the same generalform as the lower die, with thread groove-forming or groove-enteringribs 23e, 23h, 23c etc. and intervening thread-forming or guidinggrooves 24e, 24", 24 etc., the ribs beginning at a shallow height atsuccessive points along the outer edge 20 of the die, the guiding ribshaving a side contour, longitudinal rate of rise and angle of topsurface conforming to the corresponding ribs on the lower die but placedto act in the thread groove a halfturn of the work later than the lowerdie ribs. At their trailing ends, the first two ribs are raised toparticipate in tapering the point of the screw and the third one is alsoraised slightly to follow in the tapered groove bottom, while the sidesof the first two and the front side of the third are spread and the rsttwo groove bottoms are raised so that the first groove assists taperingofi the thread and the second one meshes with the tapered thread end.All of the ribs in this upper die have the ultimate pitch spacing of thethread turns and the groove turns to be formed on the screw, and thesides of all of them, including the first one 23, may have the ultimatethread-side angles, except that I prefer to make the angle on the backor upper side of the first rib, as viewed in Fig. 2, somewhat steeper ornearer to the perpendicular than the ultimate angle, for a purpose to bedescribed.

In accordance with -my present invention, the profile of the first rib19a on the lower die 12 differs considerably from that of the first rib23n on the upper die. It forms the beginning of the thread groove withless displacement of blank metal than is required for theultimategroove, especially at the front or lower side of said groove asviewed in Fig. I, and consequently the blank will not receive its fullincrement of elongation toward the tipby means of said first lower-dierib. Furthermore, this first rib is so placed in relation to the secondrib 19b that the intervening partly formed thread 'in the die groove 24to accommodate variations in circularity of the'rod stock in the blank,or variations in the diameter of the stock. For a wood screw threadhaving widely-spaced turns and a relatively wide groove bottom, thecross section of this first-entering rib, like that of the other ribs inbothfdies, is preferably a three-sided figure with a fiat top aspreviously described, but for a machine screw thread it may be two-sidedas later pointed out.

The included angle between the sides of the first lower dierib 19a isgreater thanthat of the second rib 19h', its top face is narrower andits pitch spacing with reference to the second rib is less than theultimate pitch spacing. This will be evident inobserving the effect ofthe first lower-die rib in forming the leading end or first part 25a ofthe thread groove 25 as illustrated on the left-hand side of the screwin the first three positions in Fig. 1. It will also be evident oncomparing the profiles and pitch spacing ofthe rst two lower-die ribs1,9l and 19b in Fig. 4. These views further illustrate a displacement ofblank metal by the first lower-die'rib less than the volume required forthe intimate contour of the thread groove, and in consequence, there isonly a partial increment of elongation of the blank in forming the firsthalf-turn of said groove.

The measuring action of the first `diegroove 22B, whereby it raises .orextrudes 'the proper volume of metal for the first part of the screwthread 26, with suiiicient excess to allow for subsequent densiiication,is also illustrated in these views. The leading portion 26a of thepartlyformed thread is somewhat wider at the top and its includedthread-side angle less than when the thread is fully formed, 'and saidtop may be fiattened or slightly rounded, and the first lower-die groove22a may be deepened and come to an apex so that the thread metal doesnot quite fill the bottom of the groove, as seen in Fig. 4.

Both the screw-thread groove and the thread, which were partially formedin a first step by the first-acting rib and groove of the lower dieduring that half-turn of the blank in which the forward end portions ofsaid groove and thread are made, will, by a dissimilar butcomplemental'op- `eration in a second step performed simultaneously onthe opposite side of the blank, further along said groove and thread, bewholly or partially rectified, re-formed or finished by the dissimilar'but complemental first-acting rib and groove of the upper die in thenext-succeeding halfturn, and the remaining increment of elongation willthereby be imparted to the blank without drawing out the thread metalwhich has been raised by the first die so as to leave a scanty orobliterated thread. This forging and re-forming action `of the upper dieis effected mainly or wholly by its first rib 23a, and is illustrated onthe right-hand side ofthe blank 16 in the first three positions on Fig.1, wherein it will be seen that the rectified thread-groove portion 25bmade by this first upper-die rib has been widened at bothA top andVbottom to substantially the ultimate pitch spacing, while the adjacentthread portion 26b has been narrowed at the base, sharpened at the topand densified, and its included threadside angle increased, by theforging action `of theupper die between its first and second ribs 23 and235. The thread may thus reach its ultimate contour in the secondhalf-turn of the forming operation on the work or, if desired, it may beleft slightly unfinished at the end of this second half-turn, by havinga top slightly wider than the ultimate thread, due to an angle on theback side of the first upper-die rib slightly greater to the screw axisthan the ultimate angle. and

then, by a third step complemental to the second one, be furtherre-formed and completely finished during the third half-turn, betweenthe second and third ribs 1S)b and 19c on the lower die.

Thus it will be seen that the screw groove and thread formation isentirely accomplished by the rst two ribs on the lower die and the firstrib on the upper die, the succeeding ribs on both dies serving merely tomesh into the formed thread groove, and by their guiding action on thescrew thread, to maintain the work in proper alignment between the dierollers l0 and li, with its axis parallel to the axes of said rollers.As the formation progresses toward the tip of the screw and is nearlycompleted, beginning at approximately the third screw position on Fig.l, the fric tional grip of the die ribs upon the formed screw body atthe bottom of the thread groove will have become so strong, owing to theincreasing area of contact, that the formed thread. turns 'will begin totravel faster than their contact surfaces on the dies, and the rotatingwork will begin to unscrew itself in the direction of its head end, asthough working against a stationary nut, such unscrewing beingaccompanied by some twisting of the partly-formed plastic screw uponitself as long as the screw remains unnished as indicated in the thirdposition on Fig. l. This causes the screw-head 17 to move away from thedie edges 20, as will be seen by comparing the last two positions withthe first two in Fig. l, and the rate of this axial progressionincreases as the screw is finished and nears the trailing ends of thedies. n forming screws by this invention, the diam= eter of the stockemployed may vary rather widely ior the same screw. For example, inthreading the railroad spike here illustrated, the dies shown willproduce the same thread from stock varying from t2; to We inch indiameter, the finished screw in each case having a inch thread. Wherethe blank employed is of less than that diameter, the forging action ofthe first ribs produces a wave in the blank metal on the advancing sidesof said ribs, which brings the blank out to the iull diameter. if thescrew is to be made in a relatively soft and ductile metal such as brassor copper, especially for the smaller screw sizes,

it will be unnecessary to ypre-heat the blank to increase itsplasticity.

The plain-suriaced die portions 21 are made as led es of not less thanone-half pitch width, formed on the respective dies adjacent the forwardsides of the rst ribs i9a and 2311, and spaced apart a distance equal tothe outer diameter of the screw cylinder, to limit the swelling of theblank due to forcing ahead of the` metal by these rst ribs or to reducethe blank to that diameter if it is oversize.

Fig. 6 illustrates the results of employing modied dies for rolling astandard or machine-screw thread upon a blank 16 in accordance with thisinvention, the head end of the blank being at the left and its tip endatV the right. The forging of the groove and thread is illustrated atthe right-handv or advancing end thereof, and the effect of the firstlower-die rib, whose outline is a two-sided gure, is shown on the lowerside of the blank in broken and full lines as it would occur during therst half-turn of the workat this locality. Said rib has formed a grooveportion 25u for approximately one half-turn of the work in a.preliminary shape having a sharpbottomed apex lying to the left of theultimate center pitch line of the thread groove 25, and the adjacent diegroove between the irst and second refieres lower-die ribs has measuredout the proper volurne of metal in the partially-formed first halfturn25a of the thread, with a slight excess for subsequent. densification.Said excess is accommodated pe tly by a deepening ci the die groovewhich cav this thread portion slightly out beyond the angle betw the sis ci f thread por on being less, the th-ec.d top wider than i ultimatethread. During the next-succes half-turn, as illustrf'ited for qusi'tcthe upper side or" the view, 't

ond step or operation di to the irst one widens i groove to aflatubottomed greater than the ultimate wi time rectifics reforms tnhalf-turn portion 26 between that of the and the ultimate top wi(included angle betw 5.1.1 re-formed and dimensions t the sides o thesecond in this case each stantially uniform t except for a the very hegiif desired of o is of different :mst-forma portion .fer-die lower-die i?the others, groove is "Lf than the seing uni id groove the irstntwo dierib es in mate, and form through-out, completely f lower-die ribs andthe rst-described )I claim:

l. The method of cci-few thread time f a blank which consists inapplying groove d thread forming operation upon one side ci said l2@blank and simultaneously thererf'th applying e. dissimilar butcomplement-al g Jove and thread forming operation upon the opposite sideof said blank.

2. The method or threading a screw which 12E comprises partially rollinga thread groove and the adjacent thread on a blank at the leadingportion only 0f said thread and to the full ultimate depth of saidleading portion, during a portion of a turn of the latter, therebyelongating the blank, further forming said groove and thread during theremaining portion of said turn and thereby further elongating the blank.

3. The method of threading a screw which comprises rolling a thread on ablank by dissimilar but complemental forming operations on oppositesides of the blank, causing progressive longitudinal displacement ofmaterial, with elongation of the blank, and completing the formation andpitch spacing of the thread in the leading portion thereof duringsubstantially one and onelhalf turns of the blank.

4. The method of threading a metal screw between dissimlar dies whoseformingl surfaces are confined to a pair of unlike ribs on one of thedies and a single rib on the other die, which comprises simultaneouslyimparting opposite dissimilar but complemental increments of thread andgroove shaping at successive points by the action of the respective diesin a. progressive rolling actior on 15o its length.

A 5. The method of threading a'screw which comprises measuring theproper cross-sectional area of blank material for the thread andpartially forming the thread with a side contour different than itsultimate contour, by displacement of said material, with elongation ofthe blank, in

one part of a lrolling operation, and further forming the thread to itsultimate side contour during a subsequent portion of said rollingoperation.

6. TheA method of threading a metal screw which comprises partiallyforming a thread groove and measuring the proper cross-sectional area ofblank metal for the adjacent thread, while elongating the blank, by arolling operation on one vside 'of the blank which leaves the threadwith a smaller included side, angle than the ultimate angle, and furtherforming the groove and thread by a rolling operation on the oppositesid'e of the blank which increases the included side angle of thethread, while further elongating said blank.

f 7. The method of threading a metal screw which comprises rolling athread groove and the proper measured volume of adjacent thread metal onone side of the blank, while partially elongating the blank'bydisplacing less than the rethat side, and displacing the remainder ofthe groove metal while further elongating the blank and further shapingthe thread by a rolling oper-v ation on the' opposite side of the blank,during a single turn of the blank at the leading portion of the thread.

-8. The method of threading a screw which comprises rolling -a thread'groove and the adjacent proper volume of thread metal, with the bottomof the leading portion of the groove spaced closer tc the next grooveturn than the ultimate pitch spacing, while partially elongating theblank, by a rolling operation on one side of the blank, reforming thegroove to the ultimate bottom pitch spacing and re-shaping the threadwhile further elongating the blank by a dissimilar but complementalrolling operation on the opposite side of said blank.

9. The method of threading a screw which comprises progressively rollinga thread on a blank with. the proper volume of material in the leadingportion for the ultimate thread, including a suicient excess forsubsequent d'ensication, said leading portion having not less than theultimate thread depth and other than the ultimate side contour and pitchspacing, and re-shaping and densifying said portion to the ultimate sidecontour and pitch spacing by subsequent'rolling in the same continuousoperation.

10. The method of threading a screw which comprises rolling on a metalblank a thread having substantially the ultimate proper volume of metaland not less than the ultimate thread depth in the leading portionthereof, with an included thread-side angle less than that of theultimate thread, and subsequently, during a continuation of the rollingoperation, rectifying the thread angle.

11. The method of threading a screw which comprises forming on a metalblank a thread groove of less than the ultimate area and pitchspacing ofgroove bottom in the leading portion thereof and an adjacent leadingthread portion having the proper volume of metal including an excess forsubsequent densification, an included thread-side angle less, and athread top width greater than the ultimate, While partially elongatingthe blank, by a rolling operation on one sidf of the blank, duringapproximately one halflturn of the blank, and th'en, duringapproximately the succeeding half-turn, increasing the area of thegroove, rectifying its bottom pitch-spacing rectifying the thread-sideangle, reducing the thread-top width and densifying the thread metal,While further elongating the blank, by a rolling operation on theopposite side.

12. A method according to claim l1 in which the, groove and thread arefurther rectified to their ultimate shape and dimensions duringapproximately the -third half-turn of the blank.

13. The method of threading an article of wood-screw form whichcomprises rolling a thread-groove and adjacent thread on a metal blankin successive stages at the leading ends ofsaid groove and thread whileelongating the blankin partial increments during successive approximatehalf-turns of the blank, by an initial forming operation on one side anda concurrently performed later operation different than but complementalto the iirst operation, on the opposite side of the blank, and, while soforming the blank metal adjacent the tip and during completion of theoperation, tapering off the thread and tapering down the adjacent blankmetal to form a reducedven'd on the article.

14. A method according to claim 13 in which the thread and screw bodyare carried to a sharp point at the screw tip and the excess blank metalis pinched oi by completion of the rolling operation.

, 1 15. The method of threading a screw which w ving and less than theultimate groove-bottom pitch spacing at the leading ends of said threadand groove while partially elongating the blank during a portion of aturn thereof, and rectifying said groove and thread by a dissimilar butcomplemental operation during a subsequent portion of the same turn,while further elongating the blank.

16. A methodaccording to claim 15 in which the leading portion of thegroove is formed with two sloping sides meeting at an apex and thethread diameter is extended beyond the ultimate diameter duringapproximately one half-turn of the blank, the groove is increased inarea and flattened at the bottom and the thread-side angle is partiallyrectified, the thread height reduced and the thread-top width partiallyreduced during 4approximately the next half-turn of the blank, and thethread is'further rectiiied to its ilnal shape and dimensions duringapproximately the third half-turn of the blank.

17. A pair of screw-thread forming dies, one of said dies'havlng threadand groove forming grooves and ribs and the other having thread andgroove forming grooves and rib unlike the former but complementalthereto.

18. A pair of screw-thread forming dies provided with screw formlng ribsand grooves, a rib gli:

20. A pair of screw-rolling dies having leading screw-forming ribs whichare o! different side contour and different pitch spacing in the twodies for completely forming e, thread-groove and the adjacent thread insuccessive stages, and following non-forming ribs for meshing betweenthe turns of the fnished thread and maintaining the alignment oi thework between the dies.

LEE A. mm.

